Tailings and Mine Waste Conference

Geosynthetics in closure : the underdog that could be a winner Tong, Alvin; Miskolczi, Iozsef; Ketilson, Erik 2015-10

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Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Geosynthetics in closure: the underdog that could be a winner Alvin Tong, Iozsef Miskolczi SRK Consulting (Canada) Inc., Vancouver Erik Ketilson SRK Consulting (Canada) Inc., Saskatoon  ABSTRACT Within North America, 939 mines are currently in production, 392 in closure, and 377 in care and maintenance. Additional 10,000-plus abandoned mines remain unremediated and are in state care. Producing mines often carry out various degrees of active closure activities to reduce risk and liabilities. This, along with active reclamation in operating mines, has created large demands for innovative engineering solutions to achieve productive and cost-effective closure objectives.  To that end, geosynthetic products are being widely used in a number mining and civil applications. Typical uses include material separation, liquid containment, drainage, filtration, and strength reinforcement. In particular, the mining industry has long embraced geosynthetics in applications such as heap leach pads, tailings storage facilities, and water management. Geosynthetics have also been used in civil infrastructures such as landfills, slope reinforcements, and agriculture. Interest in using geosynthetics within closure design is increasing among engineers and project proponents. Typical mine closure activities are comprised of decommissioning of buildings and reclamation of tailings facilities and waste rock stockpiles, and treatment of contaminated effluents before discharge into environment. Given suitable design criteria, risk management and site conditions, geosynthetics can be incorporated into these structures to enhance the performance of mitigative measures, reduce additional impacts, and increase cost efficiencies. However, some stakeholders and regulators seem to be reluctant to accept the use of synthetic products for such applications. This paper considers, from various perspectives, how geosynthetics can be viable components of low to moderate risk closure projects.   Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  1. INTRODUCTION Geosynthetics have long been accepted as a construction component in many containment, filtering, and reinforcement designs for mine operation facilities. Various types of geomembrane liners are used as containment in fuel and tailings facilities, as an impermeable zone in heap leach pads and dams, and as reclamation cover components. Geotextiles are used in many design applications to function as filter layers or cushioning zones to protect the critical liner beneath from overbearing material. Geonet is used in subaerial drainage applications in place of sand and gravel (e.g. between primary and secondary containments). Geogrid is widely used in foundation and slope reinforcement in road constructions and some tailings cover reclamations. Composite geotextiles are emerging technologies that combine geonet drainage capacity and drainage tubes into a geotextile to increase construction efficiencies and maximize cost benefits.  Geomembranes have been used in the mining industry for decades, predominately in operations of tailings containments and heap leach facilities. The industry has embraced a wide range of products including geosynthetic clay liners, different types of polyethylenes—in particular high density polyethylene (HDPE), and polyvinyl chloride or PVC (Breitenbach 2006). Bituminous geomembrane is available but not used as commonly compared to polyethylene products mainly due to its higher cost and incompatibility with hydrocarbons and various pH leachates.  The effectiveness of geomembranes in water and tailings containments is demonstrated at many operating mines and in several case studies. For example, the State of Nevada has regulations that require all tailings management facilities to be lined with geomembranes (NAC 445A). Layfield (2015) describes geomembrane material being used at a contaminated water storage in the US. In addition, Cole (2014) provides a case study on a uranium tailings storage facility (Cole 2014) in the US.  There are examples demonstrating the use of geosynthetics in closure applications. They show how geosynthetics can be used in progressive reclamation as part of large closure plans or in long-term permanent closure.  Same examples include the use of geosynthetic clay liners in the reclamation of an oil sand tailings pond (Athanassopoulos 2011), in the reclamation of an acid generating opening pit (White 2010), and in a number of European mine closures (Ewert 2008). In addition to mining applications, GCL and HDPE have long been adapted into landfill design for secondary containment. The U.S. Environmental Protection Agency has long accepted use of geosynthetics in municipal solid waste landfill and in closure of such facilities. This paper focuses on practicable application of geosynthetics in order to demonstrate they are viable products for use in closure plans with low to moderate risks. This assertion is made on a long-term performance prediction of geosynthetic material from existing studies, cost evaluation of case studies, and the risks evaluation of the closure plan.  2. APPLICATION OF GEOSYNTHETICS IN MINING The Canadian Mine Environment Neutral Drainage or MEND has outlined eight main categories shown in Table 1. Some of the cover types consist of some key design components that can be replaced with geosynthetics to achieve desire objectives. The typical replaceable key components are the very low permeable clay layer, filter material with controlled gradation, and coarse controlled capillary break layer.    Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  Table 1: Summary of Cover System Classifications Cover Types Function Description Geosynthetic Replacement Potential Isolation Cover Typically constructed over geochemically benign waste to prevent animal or human contact, provide erosion protect, and vegetation growth medium.  There are some very specific product designs to resist erosive conditions and high load support. They are more expensive and intended for small scale deployment.  Capillary Break Cover A composite cover that limits infiltration and prevents migration of contaminants through capillary action.  Multiple soil zones can be replaced by geocomposites and geosynthetic clay liners.  Barrier Cover A low permeable or geosynthetic liner to create a physical barrier to limit or exclude infiltration and oxygen, usually combined with an isolation cover on top to protect the barrier and facilitate vegetation growth.  There is a large selection of geosynthetic liners that could replace low permeable clay as needed.  Frozen/Thermal Cover In cold regions, waste can be engineered to integrate with the permafrost to remain frozen and prevent contamination.  There are insolation foam products available that protect frozen material from thawing. However, this type of product is expensive and typically deployed in small areas.  Store and Release Cover The cover works on the principle that infiltration is temporary retained within the cover material for later release into the atmosphere through soil evaporation and evapotranspiration.  Not applicable Reactive Cover Cover include organically or chemically active component to consume the oxygen or neutralize the acid generated by the waste.  Not applicable Water Cover A constant water cover to eliminate oxidation thru exclusion of oxygen contacting the waste. It is only applicable to low-lying waste, such as backfilled pits or relocated tailings.  Not applicable Saturated Soil Cover A variant of water cover where a soil is kept at very high level of saturation thus limiting oxidation.  Not applicable 3. STAKEHOLDER’S GENERAL VIEW OF GEOSYNTHETICS IN MINE CLOSURE The end goal of any mine closure project is the relinquishment of the mine site back to the landowner or a responsible authority, defined “as any government body empowered to approve activities associated with the mine closure process” (ANZMC 2000). Because custodianship and any potential liability could revert back to the government body or similar jurisdiction, stakeholders often have unrealistic expectations of the performance of mine closure structures. In some cases, some idealistic stakeholders (e.g. regulators) might require static structures (e.g. cover systems with a low to moderate risk of failure) to perform in excess of 2,500 years, virtually in perpetuity.  Because of this unrealistic stance on the perpetual performance, some idealistic stakeholders have the lay option that synthetic products will eventually fail where natural soil would retain its properties (and thus not fail). As a result, these stakeholders do not accept incorporation of the geosynthetics in closure applications. As mentioned before, geosynthetics are accepted to use in closure in many national and international jurisdictions. The exclusion to use geosynthetics creates inconsistency in low to moderate risks closure design, and it often goes against the current Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  engineering standard practice. Furthermore, this exclusion might eliminate economical means to remediate some of the sites. There are guidelines to assist in closure planning and design (MEND Guidelines) but there is an absence of uniform closure criteria.  With a lack of standardize closure acceptance criteria based on risk, comprehensive mine closure remains a challenge for virtually every mining jurisdiction in the world as it is an amorphous concept depending on the stakeholders and regulators (Holmes 2015). Studies exist, however, that try to provide realistic expectations and practical approaches for facilitating mine closure based on risks. These studies attempt to reason the expected design life of low to moderate risk structures in closure should be around 200 years (Logsdon 2013). This suggests that the idealistic expectation of 2,500-design life of closure structures should not be indiscriminately apply to all projects. 4. BENEFITS WITH GEOSYNTHETICS  4.1 Product Specifications and Performance Since geosynthetics are manufactured products and are tightly control and tested for quality assurance. The American Society for Testing Materials (ASTM) and Geosynthetics Institute (GRI) have established a large number of standards and procedures to test these manufactured products and installations. These standards and test methods ensure all finished products meet the desired specifications, most importantly, uniform performance across any given samples within the same products.  Many studies have been completed to evaluate the properties of geosynthetics for design and long-term performance. Over 32 technical papers in the 2014 Geosynthetic Mining Solution and 98 in the 2015 International Conference on Mine Closures were presented alone. One study provided evidence on whether drain tubes were functioning as intended under very high loads (excess of 3 MPa) to assist in design for large scale heap leach pads up to 180 m in height (Lupo 2014). Other studies indicated that geogrid in conjunction with an isolation covers can be deployed to remediate small diameter mine voids upwards of 400 m deep.  The performance and expected longevity of geomembrane under exposure or minimum isolation cover are hinged on the depletion rate of the antioxidants in the product. The antioxidants are chemical agents that prevent the polymer degraded and oxidize. Studies show that under non-exposed condition with field in-situ temperature of 20°C, a HDPE could have 200 years before the depletion of its antioxidant properties and 230 years before it reaches 50% degradation of its original properties, providing an expected service life of around 430 years (Koerner 2011). For reclamation closure cover applications, the geomembranes are likely to experience relatively low loads (<40 kPa or 2 m thick soil over liner), situated in cool in situ temperatures of <20°C that are not in direct contact with leachates. Certain types of products (e.g. low density polyethylene liner) can be more preferable due to its high flexibility and higher stress cracking resistance (Hsuan 2008).  Laboratory trials indicated that while geosynthetic clay liners do uptake some contaminants from the waste beneath due to capillary effects and ion exchange, no migration was found in the over liner above (Hosney 2012). Compacted clay liners are susceptible to freeze-thaw cycles and its hydraulic conductivity would be negative impacted (Othman 1992). Geosynthetic clay liner products are found to be less susceptible to performance degradation from freeze-thaw cycles (Kraus 1997). 4.2 Material Costs and Availability Smith (2013) provided estimates for compacted clay liner, natural soil capillary break cover, and composite geomembrane/geosynthetic clay liner covers. The unit cost estimates provided in the study encompass all cover components including preparation earthworks and surface water Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  management. The most likely cost for compacted clay liner was US$25.20/m2, capillary break was US$36.00/m2, and composite cover was US$34.50/m2. These estimates were based on the assumption that local soils that meet the design specifications are available at US$8/m2. The authors of this paper found the US$8/m2 soil unit cost to be optimistic and do not reflect the additional costs that are typically require for processing to meet design requirements.  Based on previous project experiences and public information made available to the authors of this paper, a cost comparison was done to evaluate the replacement cost of geosynthetics to natural soil material for specific key cover components (Brodie 2014). The costs are typical and the focus is on comparing costs as opposed to cost accuracy (Table 2). Table 2: Unit Cost Comparison between Geosynthetics and Natural Soil (in CAD dollars) Costs Erosion Control Low Permeable Zone Filter Zone Drainage Zone Capillary Break Zone Riprap (1m thick) Geo-cell CCL (1m thick) Geo-membrane Sand and Gravel (1m thick) Geotextile Clean Gravel (1m thick) Geonet (4 layers)(7) Sand and Gravel Composite Geonet (8) Estimated Raw material cost 15.00  18.00  11.58 (1)      6.50   11.58  4.50  11.58   22.00  11.58  12.25  Processing/ installation cost 6.00(2)  6.86 (3)    1.50(4)  4.50  11.00(5)  2.00  14.50(6)  2.00  11.00  2.00  Estimate Total    21.00       24.86    13.08    11.00    22.58       6.50    26.08    24.00    22.58    14.25  Notes: 1. The cost is based on load, haul place and dump at a distance of 3.5km. The cost is adjusted to 2015 rate based on in-house data and contracted rate in a number of closure projects.  2. Estimated blasting and screening cost. The cost is adjusted to 2015 rate based on in-house data.  3. Cost of manual installation cost plus sand and gravel backfill. The cost is adjusted to 2015 rate based on in-house data. 4. Additional compaction cost. The cost is based on in-house contractors’ quote in 2014 dollars.  5. Large scale mechanical screening cost. The cost is based on in-house contractors’ quote in 2014 dollars.  6. Screening plus washing cost. The cost is based on in-house contractors’ quote in 2014 dollars. 7. 4 layers of geonet are needed to provide approximately flow rate of 1m clean gravel. The cost is based on in-house contractors’ quote in 2015 dollars. 8. Specialized tri-planar geonet encased within 2 layer of geotextile. The cost is based on in-house contractors’ quote in 2013 dollars. The unit costs for soil material are heavily dependent on haul distance. The further the distance between the cover site and borrow, the higher the costs. The soil processing costs can be eliminated if a local borrow source is available to yield in situ material that meets engineering specifications. However, this is often found unlikely and some levels of processing are typically required.  The costs for geosynthetics are also generally related to crude prices as the majority of products are polymer based. The multiple manufacturers on the market typically keep the unit prices of the common products competitive. The biggest variable of geosynthetic product costs is transportation to project site, but the impact of this is usually minimize because of the large quantity needed for most closure covers. Subgrade and foundation preparation are considered integral to the cover installation; thus, it is not separated out specifically for geomembrane. A Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  compacted clay liner or capillary break soil cover would require subgrade preparation prior to receiving specified material, similar to geomembrane deployment.  Due to the rigorous quality control and testing of geosynthetics, these products are guaranteed to perform according to design, barring installation imperfections. The source of geosynthetics can be assumed to be always available for individual closure projects, as it is a manufactured imported material. This poses a significantly reduced risk compared to natural soils from local borrows. Unless thorough delineation studies are completed during closure planning at the local borrow sources, a risk of running out of qualified construction material exists. Additional borrow source material or processing might be required, which could compromise the design, schedule, and cost of the work. Furthermore, additional disturbance to the environment is needed to develop any borrows that are outside of the existing mine’s disturbed footprint. Based on the authors’ experience, detailed delineation studies are often not done during the closure planning stage.  4.3 The Value in of Geosynthetics in Closure It is the authors’ belief that geosynthetics are viable for use in mine closures. The idealistic view wanting closure structures to perform up to a thousand years with minimum to no repair and maintenance is unrealistic.  Consideration of closure stages, closure risks, and engineering practices suggests a planning period for management of mine waste should be normally be 200 years. The closure plan should include a semi-quantitative assessment of whether or not major changes in performance would occur between 200 and 1,000 years (Logsdon 2013). Based on the current research and developments, polyethylene geosynthetic products are predicted to perform well beyond 200 years. It is understood that monitoring, repairs, and maintenance would be required to ensure the installation is functioning as intended, whether or not the cover involves geosynthetics or not.  Established regulations to address utilization of geosynthetic in operation and permanent closure of municipal waste landfill exist. According to the Code of Federal Regulations for Criteria for Municipal Solid Waste Landfills, the post-closure monitoring and care requirement is 30 years (EPA 2011). The post-closure care period could increase or decrease depending on the remaining risks to human health and environmental impacts. With the president to deal with landfill closures, the mining jurisdictions and authorities could use the existing regulations to set up a standardized criteria framework to promote consistency and efficiency in the approval and permitting processes (Holmes 2015).  The beneficial use of geosynthetics would guarantee the key component(s) of the cover would perform uniform across the entirety of the project according to design. Construction flaws and imperfections will always pose a risk to geosynthetics, but they will also compromise natural soil construction if the work is done poorly. The ease of field quality control (e.g. air pressure testing and vacuum bell tests) increases the efficiency of quality assurance of the installations. Given the manufactured nature of geosynthetics, their utilization could significantly reduce the risks posed by natural variabilities in properties and quantities in natural soils.   It is the authors’ opinion that geosynthetics are viable products for closure projects. They could reduce the construction risks and costs, assure the performance of key design features, and reduce the overall disturbed footprint. The idealistic view to exclude geosynthetics due to the belief of their eventual total failure in 1,000 plus years can be prohibiting to reclamation of 10,000-plus abandoned sites across Canada. Risk based analyses could determine the potential impact in case of total failure. The risk evaluation and impact assessment could help in setting up mitigative measures such as setting up a bond using net present value to calculate the costs for complete reinstallation of the cover, i.e., in 1,000 years when the product completely fails.   Proceedings Tailings and Mine Waste 2015 Vancouver, BC, October 26 to 28, 2015  5. REFERENCES  [AMZMC] Australian and New Zealand Minerals and Energy Council and Mineral Council of Australia (2000) Strategic framework for mine closure, Canberra, Australia.  Athanassopoulos, C., Wells, P.S., Trinca, S., Urchik, W. (2011) The use of geosynthetics in the reclamation of an oil sands tailings pond. Technical Reference 267.  Breitenbach, A.J. (2006) Overview of geomembrane history in the mining industry. Proceeding of the 8th bi-annual meeting of the International Geosynthetis Society, September 2006.  Brodie, J. (2014) Reclaim 7.0, prepared for Aboriginal Affairs and Northern Development Canada – Water Resource Division.  Cole, J., Walls, J., Collins, R. (2014) Husab tailings storage facility containment design. Proceeding of Geosynthetics Mining Solution. [EPA] Environmental Protection Agency, (2011) Title 40 – Protection of Environment, Part 258 – Criteria for Municipal Solid Waste Landfills.  Ewert, W.F., Heerten, G., Lersow, M. (2008) Using geosynthetics for safety mine closure, closure of mining and milling residues and for groundwater protection. Proceeding of 10th International Mine Water Association Congress.  Holmes, R., Flynn, M., Thorpe, M.B., (2015) “A frame work for standardized, performance-based completion criteria for mine closure and mine site relinquishment”. International Conference for Mine Closure. Hosney, M., Rowe, K. (2012) Geosynthetic Clay Liner for Covering Gold Mine Tailings. Second Pan American Geosynthetics Conference & Exhibition.  Hsuan, Y.G., Schroeder, H.F., Rowe, K., Müller, W., Greenwood, J., Cazzuffi, D., Koerner, R.M. (2008) Long term performance and lifetime prediction of Geosynthetics. 4th European Geosynthetics Conference. Koerner, R., Hsuan, Y.G., and Koerner, G. (2011) “Geomembrane Lifetime Prediction: Unexposed and Exposed Conditions.” GRI White Paper #6, Geosynthetic Institute.  Kraus, J.F., Benson, C.H., Erickson, A.E., Chamberlain, E.J. (1997) Freeze-thaw cycling and hydraulic conductivity of Bentonitic Barriers. Journal of Geotechnical and Geoenvironmental Engineering, P229-238.  Layfield Environmental Containment (2015) Enviro Liner 6000 – Frac Water Storage Pond, https://www.layfieldgroup.com/Geosynthetics/Geomembranes/Enviro-Liner-6000-HD.aspx#item2 (accessed September 2015).  Logsdon, M.J., (2013) What does “Perpetual” Management and Treatment Mean? Toward a Framework of Determining an Appropriate Period-of-Performance for Management of Reactive, Sulfide-Bearing Mine Waste. International Mine Water Association.  Lupo, J.F. and Morrison K.F. (2005) “Innovative Geosynthetic Liner Design Approaches and Construction in the Mining industry.” Geo-Frontiers Congress 2005, Austin Texas, January 2005. ISBN: 978-0-7844-0787-5 [NAC] Nevada Administration Code, Chapter 445A.350 to 445A.447.  Othman, M.A., Benson, C.H. (1993) Effect of freeze-thaw on the hydraulic conductivity and morphology of compacted clay. Canada Geotechnical Journal, Volume 30, No. 2, P236-246.  Smith, M. and Athanassopoulos., C. (2013) An Evaluation of Engineered Cover Systems for Mine Waste Rock and Tailings. CETCO Technical Reference, TR-285.  White, W., Tong, A. (2010) Mount Washington Mine Remediation Project – Construction Phase. Proceeding 2010 Mine Reclamation Symposium.  

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